Summer weeds within the Western Australian wheatbelt - a GRDC survey

Key messages

• The most common roadside summer weed species were African love grass, windmill grass, fleabane, wild radish and sowthistle.
• The four most common species in the northern and central agricultural regions varied between years, although the four most common species in the southern agricultural region remained constant.
• Summer weed density increased in 2016 compared to 2015, due to higher December-January rainfall.

Aims

Summer weeds carry disease, impede crop sowing and utilise stored soil moisture or nutrients that may otherwise be available to the subsequent crop (Cameron & Storrie, 2014). Identifying summer weeds informs growers of the major weed issues in different agronomic regions and provides direction for future research on emerging weed species. A GRDC funded project has surveyed roadside summer weeds in the Western Australian (WA) wheatbelt in the summer of 2014/2015 and 2015/2016, and a final year of surveying will be completed in 2016/2017. This project aims to determine the prevalence and density of emerging summer weeds, and highlight the variation between years.

Method

A survey was conducted over all main roads in the WA wheatbelt during February to April 2015 and 2016. Sites were selected approximately every 10 km, where weeds were visible on the roadside. At each site, weed species were identified along a 20 m long transect. Weed density (seed head or tillers for grass weeds and plant numbers for broadleaf weeds) was determined by visual assessment. Density for each species was recorded as low (0-10 plants/m²), medium (11-50 plants/m²) or high (>50 plants/m²). At each site, the growth stage was recorded for each species (seedling/rosette, flowering, seed set) and photos were taken to allow later identification of ambiguous species. Some species could not be differentiated. For example, native grasses were placed in a single category as they are difficult to accurately identify and are not weeds. Obviously native grass species that are common summer weeds (i.e. windmill grass) were not included in this generic category. Several species could not be accurately identified as they were at the seedling or vegetative stage, like Bromus or Avena species. These unidentified weeds were identified and grouped at the genus level rather than the species level.

Results

The survey covered 246 sites over two years (Table 1). Over the entire wheatbelt, the most common species were African lovegrass, windmill grass, fleabane and wild radish (Table 1, 2). However, over the two seasons the most common species varied by region. For example, in the northern agricultural region, African lovegrass and wild radish were common weeds over both seasons. However, fleabane and couch were common in 2015, and replaced by windmill grass and green mullamulla in 2016. In the central agricultural region, African lovegrass, windmill grass and wild radish were common in both years, but fleabane was common in 2015 and roly poly was more prevalent in 2016. In the southern agricultural region, African lovegrass, windmill grass, fleabane and sowthistle were the most common weeds in both years.

The survey identified 91 different species, 48 of which were found at more than 1% of all sites over the two years (Table 2). In 2015, most species, regardless of their frequency, were present at low densities at each site. Only six species (African lovegrass, wild radish, couch, wild oats, capeweed and ryegrass) were found at high densities at any site. By contrast, in 2016, 41 species were found at high density at one or more sites. However, rainfall in December and January was higher in 2016 in all agricultural regions compared to 2015 (Bureau of Meteorology, 2016).

Table 1: The number of survey sites in each agricultural region in each year, or number of sites over the entire wheatbelt, and the frequency of the four most common weed species in each region.

Table 2: The frequency of each weed species in each year and the total frequency of each species over two years (for all species found at more than 1% of sites). Note that weed species are arranged in order of decreasing total frequency.

Conclusion

Two years of survey work have highlighted the variability of summer weeds. It is not surprising that the high summer rainfall in the summer of 2015/2016 increased the number of species present at high densities. However, it was interesting that the most common weeds found in the northern and central agricultural regions should vary between years.

Wild radish, fleabane, windmill grass, sowthistle and roly poly have been identified as problematic weeds in prior surveys (Michael et al, 2010). As a result, they are the subject of current integrated weed management projects in WA, along with Afghan melon, button grass, wireweed and caltrop (GRDC project UA00149 and UA00156). Further, stinking lovegrass, feathertop Rhodes grass, mallow and Matricaria have been highlighted as locally important weeds to research in project DAW00257. Wild radish, in particular, is common in the northern region, and has dramatically increased in the central region over the past 15 years, possibly due to the spread of resistant populations (Borger et al, 2012; Owen et al, 2015). The prevalence of wild radish in both summer and winter surveys highlights that this is one of WA’s most severe weed problems. The summer cohorts will not produce as much seed as the winter cohorts, but 2.5% of wild radish seed can remain dormant for over six years, so it is vital that no additional seed enters the seed bank (Cheam, 2006; Hashem, 2006). However, applying herbicides to wild radish cohorts in summer as well as winter will exacerbate the development of resistance. It is clear that there is an urgent need to focus research on economically desirable integrated weed management programs for wild radish, which aim to control this species throughout the year.

Some of the other major weeds identified in the survey may need further research. There is very little research on African lovegrass in WA, although there has been considerable research into the control of this species in the Eastern States, some of which is applicable to WA (NSW Department of Primary Industries, 2015). Other major weeds like couch, Afghan thistle, stinkwort, prickly paddy melon were also prevalent on the WA roadside, and may require further research for optimal control. Most of the weeds recorded in this survey have the capacity to invade cropping regions, but it is clear from these results that infestation of summer weeds depends on timing and frequency of summer rainfall. As a result, a third year of survey work is required to confirm those species that are occurring most consistently.

Acknowledgements

The research undertaken as part of this ‘Emerging summer weeds’ project (UA00149) is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC; the authors would like to thank them for their continued support. Thanks are due to Neville Chittleborough for assisting in this survey, and to Glen Riethmuller and Alex Douglas, for reviewing this work.

References

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Bureau of Meteorology (2016) Climate data online. INTERNET, available at: Climate Data Online (last accessed 14 October 2016)

Cameron J & Storrie A (2014) Summer fallow weed management. A reference for grain growers and advisers in the southern and western grains regions of Australia. Grains Research and Development Corporation, Kingston, Australian Capital Territory.

Cheam AH (2006) Seed production and seed dormancy in wild radish (Raphanus raphanistrum L.) and some possibilities for improving control. Weed Research 26, 405-414.

Hashem A (2006) Management of herbicide-susceptible wild radish population in rotational cropping. In: Symposium of wild radish and other cruciferous weeds. (ed A Cheam), 73-80. Department of Agriculture and Food Western Australia, Perth, WA.

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Owen MJ, Martinez NJ & Powles SB (2015) Multiple herbicide-resistant wild radish (Raphanus raphanistrum) populations dominate Western Australian cropping fields. Crop & Pasture Science 66, 1079-1085.